This invention relates generally, to the art of making a laminate of plural large sheets of thermobondable, laminar stock. More particularly, it relates to a laminating machine and to a process of making a laminate in a unique laminating system. The term "system" is used to refer to the laminating machine in combination with the roller means supported on tables, which roller means allow the work to be returned to an operator of the machine. The term "laminating machine" refers to a combination which comprises, tandem heating and cooling platen presses, a pair of endless loop conveyor means which support the "work" only near its opposed lateral edges and which together guide the work and position it with appropriately timed stops and starts, to a discharge station.
At least one side of one of the sheets is imprinted or otherwise marked with indiciae providing data on the card, which data are to be read, either visually by human eyes, or by a photoelectric means such as a photoelectric eye, or some other data reading means. When the laminate is to yield wallet-size (about 8.5 cm long.times.5.5 cm wide) credit cards, indiciae include a logo and/or letters spelling out the name, or otherwise indicating the issuer of the card, shapes of different color printed against a background of yet another color, a hologram, which together unmistakably identify the issuer, and also, letters spelling out the name of an individual or organization to which the card is issued, with numerals which provide an identification code, and optionally still other alphanumeric symbols which serve the purpose of providing information desired by the issuer of the card when it is used or misused for its (the card's) specified purpose. Such letters, numerals and symbols are typically embossed to raise them above the smoothly planar surface of the card, and correspondingly to indent them on the card's rear surface. Operating instructions to be left outdoors on "instructions" card for a machine, a frequently used map of a city, or of a geographical region, or any information on a card which is frequently handled, may also be provided on, or cut from, a laminate made in the machine of this invention.
The problems encountered in the production of a laminate such as the one produced herein stem from uneven temperature and pressure distribution, and particularly having to transport and store hot pressed laminates before they are cooled under pressure. The physical limitations of a sheet of thermobondable synthetic resinous material from which the laminate is made, and the requirement that the laminate maintain essentially the same sharpness of color and resolution of image as in a printed sheet of paper, after it has been subjected to aggressive heating and cooling conditions, requires that the work be treated (heated then cooled) only after it is encased between the cover plates of a thermally conductive carrier. These preconditions distinguish the task at hand from laminating plywood, forming laminates of inorganic materials which are adhesively bonded, or thermobonding a thermoplastic transparent film to a document, or heat-sealing a thermally substantially insensitive article, such as a paper document, between opposed such films.
Though a large laminate of plural sheets of thermobondable stock is technically designated the "work" in this art, it will be referred to herein as a "large laminated sheet of credit cards", and for brevity, "a laminate". A laminate is typically formed from an assembly of a core sheet, or split core sheets in congruent contact with each other, held between upper and lower transparent protective foils termed "overlays". The function of the overlays is to provide an attractive finish, and at the same time, to protect the core stock from the effect of sunlight, moisture, and to help resist wear and tear from frequent use of the card. A core sheet of plural core sheets is termed a "split core". Such a laminar assembly of a core sheet(s) and overlays is referred to as a "product pack" or "set". An assembly of such sets, typically from 1 to 10, though as many as 20 may be used if the materials to be laminated allow, is referred to herein as a "sandwich". To be "processed" (compressively heated, then cooled), a sandwich is placed in a sandwich "carrier" comprising upper and lower cover plates between which the sandwich is tightly and compressibly held. The carrier with the sandwich in it, is termed a "book".
A set typically comprises at least one sheet of core stock of uniform thickness, and often two sheets, of relatively thick core stock (relative to the "overlays") of thermoplastic synthetic resinous material suitably imprinted or, otherwise marked with indiciae, between opposed overlying "overlays" of "finish stock" or "plastic film" each overlay congruently disposed upon the exposed upper and lower surfaces of the core sheet(s). The core sheet(s) is relatively thick, from about 0.25 mm (10 mils) to 1.27 mm (50 mils), compared to the overlays. Such an overlay may be a self-supporting thermoplastic, essentially light-permeable (that is, transparent) thin sheet from about 0.00635 mm (0.25 mil) to 0.0762 mm (6 mils) thick, or a non-self-supporting thin film from about 1.25.mu. (0.1 mil) to about 6.25.mu. (0.5 mil), of a thermoplastic resinous material referred to as a "varnish" which is chosen because it is compatible with the thermoplastic core sheet stock upon which it is coated, for example by screen printing it on, to provide a layer of uniform thickness. By "compatible" is meant that, upon heating under pressure at a temperature above the Tg of the overlay, the overlay is thermally bonded to the core stock, so that there is no phase separation and no discernible boundary therebetween. The outer surfaces of the core stock may also be coated with an adhesive which is compatible with both, the core stock and the polymeric material of the overlay.
In addition, each sandwich, whether of a single or plural sets, necessarily includes a pair of flexible, elastomeric sheets of uniform thickness, referred to as "lamination pads" placed above and below the upper most and lowermost set in a sandwich. The lamination pads perform a critical function, namely, they distribute the pressure exerted by the platens essentially uniformly over the surface of the overlays and core sheet, and are typically placed between the cover plate and the first polished plate. Insufficient pressure over even a small area, from 0.1 mm.sup.2 to 1 mm.sup.2 will cause "lakeing" namely leaving a depression in the surface, a defect which may be visually observed by tilting the surface of a laminate at an angle towards a beam of light. Too great a pressure, on the other hand, will cause "running" evidenced by the edges of the laminate being advanced past the original boundaries of the core sheet, and bleeding of the colors of the inks used to print the indiciae on the core sheet. Both distortion and bleeding of colors is also encountered when the hot laminate is held in the book too long.
Though a sandwich of a single set may be processed, it is impractical to process individual sets separately. Therefore, typically, multiple sets are simultaneously processed by interleaving each set with a thin metal plate from 0.49 mm (20 mils) to 0.98 mm (40 mils) thick, typically 0.635 mm (25 mils), having highly polished or microfinished surfaces which transfer their finish to the overlays. In practice, a sandwich may include from 5 to 20 sets having from 6 to 21 polished plates, respectively. Even more so than with a single set, it is critical that a chosen set pressure, typically about 689 Kpa (100 psi) in the heating press, and about 2756 kPa (400 psi) in the cooling press, will be exerted essentially continuously on the multiplicity of sheets in a sandwich, after the carrier is biased against the upper stationary platen, and that this pressure be uniformly distributed. By "uniformly distributed" is meant that there is no difference in pressures exerted per unit area measured at spaced-apart, coplanar portions in either the uppermost surface of a set, or, in portions in the lowermost surface of a set, sufficient to cause a visually unacceptable finish on the processed laminate.
Each sandwich is placed between rigid upper and lower cover plates, one vertically aligned above the other, having mutually facing planar surfaces for contacting the upper and lower surfaces of a set, and having lengthwise and widthwise dimensions exceeding those of the sandwich. Such a laminar assembly is referred to as a "book". Each book is compressively heated, then cooled, each of these steps being carried out under pressure, to form the laminate. The cover plates are biased, one against another, under a pressure which is maintained essentially constant during the heating cycle, and is exerted in a direction orthogonal (at right angles) to the direction in which the book is to be translated on a conveyor. One skilled in the art will appreciate that the specifications for the cover plates will depend upon the demands of rigidity, which in turn will depend upon the pressures to be experienced in the press. For obvious reasons, metals are chosen with high thermal conductivity and a high modulus of elasticity in shear or a high coefficient of rigidity. Since metals such as aluminum have high thermal conductivity but a lower coefficient of rigidity than steel, a common preferred choice is a cover plate from about 1 cm (0.375") to 2 cm (0.75") thick of aluminum, or bronze where the cost can be justified.
Instruction cards, and credit cards in particular, are issued by the tens of thousands, and sometimes the million, to persons and organizations many of whom may, or may not, be benefitted by the receipt and use of the card. Since the cards are generally to be made in a single "production run", there is a high premium on the rate at which a "book" can be processed.
Until relatively recently, a book was processed in conventional laminating devices comprising upper and lower "dual-function" platens (so termed because they could both heat and cool the "work"), one or both of which were provided with hydraulic motive power to move at least one of the platens in the vertical direction, so that the lower face of the upper platen and the upper face of the lower platen tightly held and compressed the "work" therebetween. The dual-function platens were each provided with internal or external heating means, which in conjunction with appropriately programmed electrical timer means, enabled the platens to be synchronously heated to the desired temperature at which satisfactory lamination was effected; and, each platen was also provided with internal or external cooling means which enabled the platens, after each set in each book was laminated or otherwise treated, to be synchronously cooled to the desired temperature at which the "book" could be discharged from between the cold platens. However, the loss of time inherent in having to cool a heated platen after a "book" was processed in a dual-function platen press, was unacceptable when a large order or "run" of credit cards had to be delivered to a purchaser (eventually, the issuer of the credit cards) under the pressure of an unrealistically short deadline. Some of the problems endemic with dual-function presses equipped with multiple platens are addressed in U.S. Pat. No. 3,241,189 to Siempelkamp.
This problem, namely, the need to deliver a very large number of satisfactorily laminated credit cards in a short time, implanted an expectation of high productivity in prospective purchasers and users of laminating presses for credit cards, which expectation resulted in this characteristic becoming the sine qua non specification to be met by a laminating machine which was acceptable to such a purchaser or user.
To this end, laminating presses have been constructed with the emphasis on economy and ease of operation, and speed. For example, in U.S. Pat. No. 4,543,147 to Noto et al, the discovery that the compressive heating of a book could be interrupted, led to the use of a bi-level tandem multiple daylight opening presses in which the platens intermediate the ram platen and the fixed upper platen, were interconnected for identical up-and-down movement. Each press has one daylight opening for heating and the other daylight opening (or "daylight" for brevity) in the same press, for cooling. After treatment in one (the upper level, shown) of the machine, books are stacked in a stacking press, and on their return, the books are treated in the lower level, by being sequentially precooled in the same press they are finish-heated (a very large temperature difference .increment.T), then finish-cooled in the same press they were pre-heated (a smaller, but still large .increment.T).
The net effect is to reduce the cycle time for a book. But their requirement that there be vertically spaced apart entrance and discharge stations, so that the press may be operated by a single operator stationed at the entrance station of the press, resulted in having to use a "stacking press" which provided no function other than to return the heated book to the first of two tandem cooling presses. In addition to the cost of an additional press, the stacking press provides a substantial delay between transfer of a book from the heating to the cooling zones which is generally deleterious in making laminates which are highly sensitive to the heating and cooling cycles, as is, in particular, the laminate of this invention.
Moreover, the use of heating and cooling cycles in the same press require that the same pressure be used for heating and cooling. This is incompatible with maximizing the "window of lamination" which dictates one use the lowest practical pressure in the heating cycle to minimize "bleeding" of colors and distortion of lines, and, the highest practical pressure in the cooling cycle to "iron out" microscopic and submicroscopic surface irregularities when the polymer as cool as will permit doing so, again to minimize distortion. Making most laminates demands utilizing the "window" to allow one to meet the quality criteria for a no-defect, high-gloss, "ultra-finish".
Further, a finish-heating cycle in the upper daylight of the same press in which the lower daylight is used to precool a book, requires the press to operate reliably despite an inordinate temperature gradient .increment.T which is only slightly less in the first press where the upper daylight is used to preheat, and the lower daylight to finish-cool, than in the second press.
A different approach utilized a "stack" of several "books" which could be simultaneously loaded into successive heating and cooling presses, each of which comprised multiple platens, referred to as multiple opening presses.
A press with a single pair of platens is referred to as a "single daylight opening" (or "SDO") press, or a "single daylight" (or "SD") press, and a press with multiple daylight openings is referred to as a multiple daylight opening ("MDO") press, or, a multiple daylight ("MD") press. Such MDO presses are commercially available from Robert Burkle Gmbh & Co., West Germany; and, Oakwood Design, England; inter alia. Because these presses rely on each book in an opening of a free standing MDO heating press "shunting" another book into its subsequent position in a corresponding opening of a MDO cooling press, any significant misalignment of the shunting and shunted books in the lateral plane, results in a problem. Further, such prior art MDO presses require the use of a loading device which simultaneously pushes multiple books to feed the books from a storage magazine to the MDO heating press, and thence to the cooling press, and it is difficult to maintain the path of the books in a longitudinally axial path.
Against this background of concentrated effort devoted to solve the problems endemic to the operation of a laminating machine for a set of thermoformable synthetic resinous sheets, for making laminates of all types and sizes including plywood, identification cards and circuit boards, this invention provides easily available components which in combination can be adapted for any of the foregoing purposes. With the use of MDO presses, and ganged heating and cooling presses, the cycle time for processing a book may be lowered to be competitive with any known machine.
As will be described in greater detail below, the basic embodiment of the laminating machine and the process for making laminates simply and efficiently, may be adapted to a two-step heating and two-step cooling machine (referred to as the "two-step" machine).
In another embodiment referred to as the "MDO machine" multiple books, in a "stack" of spaced apart books, are laminated concurrently, then cooled and discharge concurrently, using only one MDO heating press and only one MDO cooling press in a two-press mode. Since each daylight requires its own fixedly mounted, edge-conveyor means, it will be evident that it is essential that the endless loops be tightly looped, that is, returned with a radius less than about 7.5 cm if the travel of the ram is 7.5 cm, or the conveyors will interfere with the travel of the platens. Further, such an MDO machine requires that provisions be made to load the vertically spaced apart daylights from an appropriate loading-conveyor; and, unload vertically spaced apart discharge stations with an appropriate discharge-conveyor. Details for the construction and operation of tandem MDO heating and cooling presses in an MDO machine are provided herebelow for those instances when high production in a short time is demanded.
For maximum production, still another embodiment, referred to as the "ganged tandem MDO machine" is used. Details for the construction and operation of ganged tandem MDO heating and cooling presses in a ganged MDO machine are provided herebelow for those instances when maximum production in a minimum time is demanded.